1. Field of the Invention
The present invention generally relates to a belt driving mechanism, an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions, that includes the belt driving mechanism, and a belt position adjustment method.
2. Discussion of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include an image carrier on which an electrostatic latent image is formed, a development device to develop the latent image with developer, a transfer unit to transfer the developed image (toner image) onto a sheet of recording media, and a fixing device to fix the toner image on the sheet with heat and pressure.
Fixing devices typically includes a heat source, a heating member (e.g., heating roller) to transmit heat from the heat source to the toner image formed on the sheet, and a pressure member (e.g., pressure roller, pad, etc.) that presses against the heating member, thus forming a fixing nip with the fixing member. Alternatively, the pressure member may press against the heating member via a fixing member.
Such electrophotographic image forming apparatuses typically use belts and driving mechanisms thereof for various purposes in addition to power transmission and transportation.
For example, certain known electrophotographic image forming apparatuses use a belt as the fixing member or heating member because the smaller heat capacity of the belt makes it possible to reduce the warm-up time of the fixing device, that is, the time period required to heat the fixing member to a target temperature. Shortening the warm-up time of the fixing device is important to reduce waiting time of the image forming apparatus from when users turn on the apparatus, particularly in the morning, to when and the apparatus is ready for image formation.
Certain known fixing devices use, as the fixing member, a belt (hereinafter “fixing belt”) stretched around at least two rotary members (e.g., a first rotary member and a second rotary member), which together form a belt driving mechanism. The first rotary member has a lower thermal conductivity and is disposed facing a third rotary member via the belt, thus forming a fixing nip. The heat source is disposed inside the second rotary member. As the heat source, induction heating units are known in addition to heaters. The induction heating units typically use material having a smaller heat capacity as the fixing member and heat the fixing member through an inducting heating method. Hereinafter fixing devices using the fixing belt are referred to as “belt fixing devices”.
Because the fixing belt having a smaller heat capacity can be heated relatively quickly, the warm-up time of such fixing devices can be shorter. In certain known fixing devices, the fixing belt is wound around the heating roller inside which the heat source is provided, and a tension roller, serving as the pressure member, is pressed against an outer surface of the fixing belt so that a relatively large area of the fixing belt contacts the heating roller. Thus, a large amount of heat can be transmitted to the fixing belt in a shorter time period, thereby reducing the warm-up time of the fixing device.
The belt driving mechanism should prevent any positional deviation in a width direction or thrust direction of the belt from occurring or correct it when it occurs, and it is known that positional deviation in a thrust direction of the belt tends to occur in image forming apparatuses. For example, in certain known image forming apparatus, an intermediate transfer belt is wound around multiple rollers including an upstream roller disposed on an upstream side in a direction in which the intermediate transfer belt rotates, and the intermediate transfer belt is biased to a first end of the upstream roller in a thrust direction of the upstream roller. A stopper is provided on an end portion of the intermediate transfer belt opposite the first end of the upstream roller. With this configuration, the intermediate transfer belt is biased in a predetermined direction and the stopper provided on the intermediate transfer belt contacts the other end of the tension roller to set the position of the intermediate transfer belt in the width direction. It is known that the position of the belt deviates in the width direction when pressing force of rollers each pressing against the belt is not balanced in the thrust direction.
It is to be noted that a disengaging unit to disengage one of the rollers pressing against each other from the other roller to adjust the pressing force of the rollers is known. For example, in certain known fixing devices in which the pressure roller directly presses against the fixing roller without any belt, the pressure roller is disengaged from the fixing roller when sheets are jammed between the two rollers to facilitate removal of the sheets therefrom. Although, typically, a cover of the apparatus is opened and then a lever is operated to move one of the fixing roller and the pressure roller, an arrangement in which the image forming apparatus includes a disengaging unit using a cam can be used. The disengaging unit using the cam is advantageous in that users do not need to open the cover. Additionally, the operational life of the fixing roller can be extended by simply operating the disengaging unit to disengage the fixing roller from the pressure roller when sheets do not pass therebetween. However, this disengaging unit does not consider the balance of the pressing forces between the two rollers.
In view of the foregoing, there is a need to prevent, reduce, or correct the positional deviation of the belt with a simple configuration which known methods fail to do.